code: def _build_loss_function(self):
      self.loss = Graph2SeqLoss(
          ignore_index=self.vocab.out_word_vocab.PAD,
          use_coverage=self.use_coverage,
          coverage_weight=0.3,
      )
  
imports: import os
  import resource
  import numpy
  import torch
  import torch.optim
  from torch.utils.data import DataLoader
  from torch.utils.tensorboard import SummaryWriter
  from graph4nlp.pytorch.models.graph2seq import Graph2Seq
  from graph4nlp.pytorch.models.graph2seq_loss import Graph2SeqLoss
  from graph4nlp.pytorch.modules.evaluation import BLEU
  from args import get_args
  from build_model import get_model
  from dataset import IWSLT14Dataset
  from utils import WarmupCosineSchedule, get_log, wordid2str
label_desc: Use image node_id 8 for calling the NMT obj's underlying member method code with example usage: obj._build_loss_function() without return types